The present invention relates to gas turbine engines and, more particularly, to thermal shield assemblies for gas turbine engines which include cooling elements for conveying air to one or more disk assemblies.
In a gas turbine engine of the type used in jet aircraft applications, first and second stage turbine disks support turbine blades which require air cooling under normal operating conditions. This is accomplished by pumping air into a confined space or cavity between the first and second stage disks, then directing the air from that space to passageways formed in the turbine blades themselves.
This interstage cavity is defined by the first and second stage disks, the shaft on which they are mounted and a thermal shield, which is located radially outwardly of the shaft. The thermal shield is generally cylindrical in shape and is attached at its ends to the first and second stage disks. Typically, the thermal shield is bolted to the disks. However, a disadvantage with such a bolted connection is that it does not allow for expansion and contraction of the thermal shield relative to the disks in response to thermal changes. This rigid connection therefore creates high thermal stress concentrations in the thermal shield which significantly shorten the useful life of the shield. Further, such bolted connections, which may require as many as 80 bolts per disk, are time consuming to secure.
Similarly, the thermal shield assembly typically includes a spacer impeller which extends between the first and second stage disks and is being bolted at its radially-inner periphery to the second stage disk, and at its radial outer periphery to the first stage disk. The spacer impeller is formed by two juxtaposed disks which are divided by ribs into a plurality of spoke-like, radially-extending passageways. The impeller ducts cooling air in the chamber radially outwardly and forwardly toward the first stage disk which would otherwise follow a pressure gradient favoring the second stage disk.
A disadvantage with this type of disk impeller structure is that the bolted connections at the inner and outer peripheries do not allow for the expansion and contraction of the impeller relative to the first and second stage disks.
The thermal shield assembly also includes an annular, disk-shaped bore which is connected to and extends radially inwardly from the thermal shield adjacent the spacer impeller. The bore is required in order to add hoop strength to the shield to prevent buckling and other deformation of the shield during operation of the turbine engine.
A disadvantage of such bore designs is that pressure gradients within the area bounded by the thermal shield between the first and second stage disks causes the bore to deflect rearwardly toward the second stage disk, thereby bending the thermal shield.
Accordingly, there is a need for a thermal shield assembly which is connected to the first and second stage disks such that expansion and contraction of the thermal shield and spacer impeller resulting from thermal stresses relative to the first and second stage disks is minimized. Further, there is a need for a thermal shield assembly in which the thermal shield bore resists deformation in response to pressure gradients without adding expensive and relatively heavy reinforcing members.